AISC LRFD 1.pdf

Comm. C3.] STABILITY BRACING 195age of leaner columns. Less conservative methods are given in ASCE Task Committeeon Effective Length (1997). The term,∆ΣP⎛ oh ⎞u ⎜ ⎟⎝ ΣHL⎠is a constant for all rigidly connected columns in a story and is the same term used inEquation C1-4.For the story buckling approachP æeSPöuK ¢ =(C-C2-6)P çu èSP ÷e 2 øwhere( ΣPu / ΣPe2 ) is the same term found in Equation C1-5 and is a constant for allrigidly-connected columns in a story.The value of P n calculated using K by either method cannot be taken greater than P nbased on sidesway inhibited buckling. Additional simplified methods were given inthe previous edition of this commentary. Although they are not repeated here, theyare equally valid within the limitations placed on them in that edition. A comparisonof the influence of those methods may be found in Geschwindner (1994).The theoretical K-factors that are less than 1.0 (Cases (a) and (b) in Table C-C2.1and the sidesway inhibited alignment chart in Figure C-C2.2a), are based on theassumption that there is no relative lateral movement of the ends of the column.When bracing is proportioned by the requirements of Section C3, K equal to 1.0should be used, not values less than 1.0, because a small relative movement of thebrace points is anticipated.C3. STABILITY BRACING1. ScopeThe design requirements consider two general types of bracing systems, relativeand nodal, as shown in Fig. C-C3.1. A relative column brace system (such as diagonalbracing or shear walls) is attached to two locations along the length of the columnthat defines the unbraced length. The relative brace system shown consists ofthe diagonal and the strut that controls the movement at one end of the unbracedlength, A, with respect to the other end of the unbraced length, B. The diagonal andthe strut both contribute to the strength and stiffness of the relative brace system.However, when the strut is a floor beam, its stiffness is large compared to the diagonalso the diagonal controls the strength and stiffness of the relative brace. A nodalbrace controls the movement only at the particular brace point, without direct interactionwith adjacent braced points. Therefore to define an unbraced length theremust be additional adjacent brace points as shown in Figure C-C3.1. The two nodalcolumn braces at C and D that are attached to the rigid abutment define the unbracedlength for which K = 1.0 can be used. For beams a cross frame between two adjacentbeams at midspan is a nodal brace because it prevents twist of the beams only at theparticular cross frame location. The unbraced length is half the span length. Thetwist at the ends of the two beams is prevented by the beam-to-column connectionsat the end supports. Similarly, a nodal lateral brace attached at midspan to the topLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION